var dustObj = function(){
	this.x = [];
	this.y = [];
	this.amp = [];
	this.NO = [];

	this.angle;
}
dustObj.prototype.num = 30;
dustObj.prototype.init = function(){
	for (var i = 0; i < this.num; i++) {
		this.x[i] = Math.random() * canWidth;
		this.y[i] = Math.random() * canHeight;
		this.amp[i] = 20 + Math.random()*25;
		this.NO[i] = Math.floor(Math.random()*7);//[0,7)
	}
	this.angle = 0;
}
dustObj.prototype.draw=function(){
	this.angle += deltaTime * 0.0008;
	var l = Math.sin(this.angle);
	for (var i = 0; i < this.num; i++) {
		var no = this.NO[i];
		ctx2.drawImage(dustPic[no],this.x[i]+l*this.amp[i],this.y[i]);
	}
}